Issue 41

Y. Nadot et alii, Frattura ed Integrità Strutturale, 41 (2017) 220-226; DOI: 10.3221/IGF-ESIS.41.30 224 3. In order to take into account the variable amplitude loading, a non-linear cumulative damage theory is used. This theory is based on the ‘Damage Curve Aproach’ and already presented in [1]. A nonlinear cumulative damage approach is necessary because the spectrum is such that the overload induces strong non linearity. Fig. 8 presents the results obtained using the equivalent fatigue stress (or Fatigue Indicator Parameter) proposed by Vu and computed at the local scale without any gradient correction and considering the identification procedure presented in Fig. 9. The parameters (  1 ,  2 ,  3 and  ) are obtained directly through some of the fatigue tests conducted on the welded structure so that the parameters contain implicitly the information related to the welded material (geometry, heat affected zone and residual stresses) and for a given smooth gradient (so that we suppose that this gradient can be neglected). Results recorded in Fig. 8 show that the local values obtained at each initiation site are very different from a load case to another so that it seems that the local approach is not the one to be used for this context. This is the reason why the WSG approach presented in Fig. 7 is proposed. The identification of the gradient parameter is presented in Fig. 9 and the gradient material parameter obtained is equal to 51  m. Fig. 10 presents the results obtained with the WSG approach compared to the local one. It is clear that the WSG approach is better than the local one, even if the error can goes up to 65 %. Figure 7 : Measurement of the stress gradient related to the local geometry. Figure 8 : Fatigue stress (Vu criterion) computed at the hot spot for constant amplitude loading.